Quote: have ofn-tile in /.gimp-2.8/plug-ins,. How do I run it please?
Bottom of the Layer menu Save-as-Tiles make sure the image is in multiples of tile size. Try with the defaults first. Look at the help for more info.
Mosaics, especially large mosaics is a subject I had an interest in, so a video demo (is quick ) with the first minute, splitting up an image using Ofnuts ofn-tiles plugin.
https://youtu.be/EOy1L0EZ9vw
Stop there if all you need is a stack of numbered tiles. The rest, 5 minutes or so, is putting them back together.
Lots of ways to do this, many probably easier. For very large graphics you need NIP2 but using Ofnuts arrange-layers-0.2.py it is the same concept but on a small scale. Join tiles into rows, join rows to complete.